Section 7 the earth's crust, rocks, minerals

I. The topmost layers of the earth are called the earth's crust. It is believed that at one time the earth was molten (liquid or almost liquid), and it is still hot enough inside to become molten under certain conditions¹. It is believed that some of the molten material cooled to form the solid crust of the earth. Any material which was formed from liquid rock is called igneous rock (Latin "ignis" means "fire"). Igneous rocks are of two main kinds: extrusive (effusive) and intrusive.

During the long periods of geologic time, igneous rocks were eroded (worn away). The forces of erosion carried away particles of eroded rock — some coarse, some fine — and set them down in layers, either on dry land or under water. In time², some of this material was buried under mud and subjected to high pressures. As a result, it changed into another type of solid material known as sedimentary rock. Igneous or sedimentary rocks were often subjected to heat, pressure, or movement, as a result of which³ their original qualities were changed in some way⁴. Thus, new materials known as metamorphic rocks were formed. (The word "metamorphic" comes from two Greek words meaning "changed form".) About three-fourths of the surface of the land is now covered with sedimentary rock, forming a layer rarely more than 1 mile in thickness. This layer lies over the igneous and metamorphic rocks of the crust of the earth.

II. The crust comprises an insignificant fraction of the earth's total mass. Its thickness and composition are not constant. The thickness of the crust under the continents averages about 40 km, that under most of the mountains averages 60 - 70 km. Under the oceans the crust is much thinner, being only about 5 – 10 km thick. The continental crust consists of light rocks such as gneiss, granodiorite and granite, and the oceanic crust is made up of dark rocks such as basalt (see Fig. 3).

F ig. 3. The Earth’s Crust under Mountains, Plains and Oceans

In its widest sense, the term "rock" includes all the solid constituents of the earth's surface, whether solid (such as granite), granular (such as sand and gravel), or earthy (such as clay). All rocks are composed of minerals. Minerals therefore are the unit constituents of the earth's crust, and as such they are of great interest and importance to the geologist. We should, therefore, start a detailed study of the earth's crust with the minerals.

A mineral is a naturally occurring substance having definite chemical and physical properties. Minerals are of many kinds. Each has its own chemical composition and atomic structure and tends to form crystals whose shapes are determined by this structure. Minerals have long been classified on the basis of chemical composition and crystal form. Of the more than two thousand minerals that have been recognized and described only about twenty are abundant constituents of the earth's crust. Most of these can readily be identified at sight by anyone who will make a careful study of their ordinary physical properties.

Notes

¹ under certain conditions — при определенных условиях

² in time — со временем

⁸ as a result of which — в результате чего

⁴ in some way — некоторым образом, в некотором отношении, до некоторой степени

Words

a) Familiar words:

atomic [əˊtɔmɪk], basalt [ˊbæsɔ:lt, ˊbæsəlt, bəˊsɔlt], basis [ˊbeɪsɪs], to classify [ˊklæsɪfaɪ], constant [ˊkɔnstənt], crystal [ˊkrɪstl], effusive [ɪˊfju:siv], to erode [ɪˊroud], erosion [ɪˊrouʒən], extrusive [eksˊtru:sɪv], gneiss [naɪs], granite [ˊgrænɪt], granodiorite [ˏgrænəˊdaɪəraɪt], granular [ˊgrænjulə], intrusive [ɪnˊtru:sɪv], metamorphic [ˏmetəˊmɔ:fɪk]

b) abundant [əˊbʌndənt] а обильный, распространенный

average [ˊævərɪʤ] v в среднем равняться, составлять

believe [bɪˊli:v] v полагать, думать

bury [ˊberɪ] v погребать, хоронить

carry away v уносить

clay [kleɪ] n глина

coarse [kɔ:s] a крупный, грубый

coarse-grained крупнозернистый, грубозернистый

cool [ku:l] v остывать, охлаждаться

composition [ˏkɔmpəˊzɪʃən] n состав (структура, текстура)

cover [ˊkʌvə] v покрывать

determine [dɪˊtə:mɪn] v определять

erode [ɪˊroud] v разрушать, эродировать, выветривать, размывать

erosion [ɪˊrоuʒən] п эрозия, выветривание, размыв

fine [faɪn] n мелкий

fine-grained мелкозернистый, тонкозернистый

fire [ˊfaɪə] n огонь, пламя

force [fɔ:s] n сила

fraction [ˊfrækʃən] n доля

gravel [ˊgrævəl] n гравий

identify [aɪˊdentɪfaɪ] v опознавать, определять

igneous [ˊɪɡnɪəs] а магматический, изверженный

inside [ˊɪnˊsaɪd] adv, a, n внутри; внутренняя часть; внутренность

insignificant [ˏɪnsɪɡˊnɪfɪkənt] a незначительный, ничтожный

kind [kaɪnd] n вид, тип

layer [ˊleɪə] n слой, пласт

lay (laid, laid) down v откладывать,

класть

lie (lay, lain) [laɪ] v лежать

liquid [ˊlɪkwɪd] a, n жидкий; жидкость

mean [mi:n] v означать, значить

melt [melt] v 1. плавить(ся), расплав-ляться; 2. таять

molten [ˊmoultən] а расплавленный

mud [mʌd] n ил, мелкая порода

particle [ˊpɑ:tɪkl] n частица, крупица

pressure [ˊpreʃə] n давление

property [ˊprɔpətɪ] n свойство

quality [ˊkwɔlɪtɪ] n качество, свойство, особенность

rarely [ˊrεəlɪ] adv редко

readily [ˊredɪlɪ] adv легко, быстро (охотно)

recognize [ˊrekəɡnaɪz] v 1. узнавать, распознавать; 2. признавать

sedimentary [ˏsedɪˊmentərɪ] a оса-дочный

sense [sens] n смысл, значение

shell [ʃel] n 1. оболочка; 2. раковина

to set down = to deposit = to lay down

still adv (все) еще, до сих пор, по-прежнему

subject [səbˊʤekt] v подвергать (воз-действию, влиянию)

tend [tend] v стремиться, иметь тенденцию

term [tə:m] n термин

thick [θɪk] a I. мощный, толстый; 2. густой, плотный

thickness [ˊθɪknɪs] n толщина, мощ-ность

thin [θin] а тонкий

unit [ˊju:nɪt] a, n 1. индивидуализи-рованный, химически однородный; 2. единица